TABLE OF CONTENTS Information can be carried by an optical beam only if the beam is modulated. There are many ways to modulate the laser emission: output wavelength, frequency, intensity, etc. Intensity modulation is used most often because it is well adapted to digital communications and relatively simple to implement. The two forms of intensity modulation are external modulation and internal modulation. External modulation can be achieved by a mechanical wheel such as a compact disc, or by an electro-optic modulator that changes the optical density or index of refraction of the propagation path, to mention two possibilities. In some applications, the laser beam is not optically modulated at all, internally or externally. An important example of this use is the laser pump at 980 nm for Er 3+ optical fiber amplifiers in optical communications.
Internal modulation is accomplished by modulating the drive current of the laser. Current modulation has the advantage that it is both simple and economical to implement. The disadvantages of current modulation are related primarily to transient effects associated with turning on or turning off the laser. Some of the main difficulties are chirping and self-pulsations. The laser chirp refers to the change of the laser output wavelength with time as the laser is being pulsed on or off. The chirp may be large enough to increase the communications error rate beyond acceptable limits. Self-pulsations are the result of...
